Implement Service Context/Runtime for io.systemd.Unit.List (#42098)

The PR implements the following objects + tests for
io.systemd.Unit.List:
- ServiceContext
- ServiceRuntime

It's hopefully the last PR of the long sequence of:

* https://github.com/systemd/systemd/pull/37432
* https://github.com/systemd/systemd/pull/37646
* https://github.com/systemd/systemd/pull/38032
* https://github.com/systemd/systemd/pull/38212
* https://github.com/systemd/systemd/pull/39391
* https://github.com/systemd/systemd/pull/41980
* https://github.com/systemd/systemd/pull/42057